Hi Ian,
The usual method of ranking members based on quality rather than quantity of contributions used by most forum boards is to either/or have the staff rate specific posts and posters based on merit rather than post count, or enable individual post/thread rating by the membership. Both are possible with VBulletin.
There is the danger in the latter case of nepotism among "pals" boosting one another's rating, but the amount of votes given to any one individual by another, specific member can be set to a limit within a prescribed time frame.